Light emitting diodes (LEDs) for use as indicators are well know. LEDs have been used extensively for this purpose in consumer electronics. For example, red LEDs are commonly used to indicate that power has been applied to such devices as radios, televisions, video recorders (VCRs) and the like.
Although such contemporary LEDs have proven generally suitable for their intended purposes, they possess inherent deficiencies which detract from their overall effectiveness and desirability. For example, the power output of such contemporary LEDs is not as great as is sometimes desired. This limits the ability of contemporary LEDs to function in some applications, such as providing general illumination, e.g., ambient lighting. Even high power contemporary LEDs do not provide sufficient illumination for such purposes.
Contemporary LEDs are also less efficient than desirable, thus making their use for general illumination less attractive. High power contemporary LEDs are even less efficient than standard LEDs, thus making high power LEDs even less attractive for some applications.
Attempts to overcome the limitation of contemporary LEDs associated with insufficient power output include the use of multiple LEDs which are ganged so as to provide the desired illumination. However, the use of multiple LEDs is more expensive than desired and inherently increases the size or volume of the illumination device, making it unsuitable for some applications.
AlInGaN based LEDs have attracted much attention in recent years due to their high potential for use as a solid state light source which is suitable for replacing traditional incandescent and fluorescent lighting. Although still not as efficient as desired, the efficiency of contemporary LEDs has been improved so much in the last decade that it is now higher than the efficiency of the common tungsten lamp.
LEDs produce light which is monochromatic. Thus, they are typically not suitable for general illumination, for which white light is generally desirable. However, by combining a blue AlInGaN LED with yellow phosphors, white light can be produced. This approach is now being used extensively in the manufacturing of white LEDs.
Nevertheless, white LEDs are more extensively used in backlight applications for liquid crystal displays (LCDs) than for general illumination. This is more due to the cost of making AlInGaN LEDs than to performance considerations. Ultimately one needs to optimize both cost and performance, so that LEDs can be competitive with respect to traditional light sources.
One issue relating to LEDs is that their efficiency can be adversely affected by heat generated within the device itself. This limits the amount of electrical power that can be used to drive an LED, and thus results in a limitation on maximum output optical power from an LED since the amount of light that can be generated is roughly proportional to the input electrical power.
The fundamental reason for lower efficiency caused by heat is due to temperature rises in the LED die. Higher operating temperatures not only degrade the light output efficiency, but also substantially reduce the life of the LED. Since heat generation in the LED is unavoidable, scientists have been trying to reduce the temperature rise by improving the heat removal rate. This can be accomplished by placing a heat sink close to the active region of the device and by choosing high thermal conductivity material for the heat sink.
Another approach is to use larger device size so that both the contact area to the outer thermal contact, as well as the total heat capacity, increases. For the same amount of heat generated, a larger device will remain cooler than a smaller device, thereby facilitating operation with higher input power. Of course, the higher the input power is, the higher the output optical power will be. Therefore, for a larger LED chip the total power that can be delivered from a single device increases and a cost saving is expected, since one large size device replaces several small size ones.
Referring now to FIG. 1, a simplified schematic of an AlInGaN on sapphire LED is shown. The most commonly used device size for an AlInGaN on sapphire LED is about 300×300 microns. The device is normally operated at 20-30 milliamps and 3.5 volts. Some contemporary designs may have different geometry, but the dimensions of the sides are approximately similar, i.e., the aspect ratio between the two sides of a contemporary LED is approximately 1. This is mainly due to the size of the conventional LED package, wherein a cup shaped recess is configured for an LED chip to mount therein and has a dimension of approximately 400 micron in either round or square.
Referring now to FIG. 2, a simplified schematic of a contemporary large size (high power) LED chip is shown. Recently, such large size AlInGaN based LED chips have become readily available. The dimension of these large size chips is about 1000×1000 microns. Usually, the actual device area (excluding the bond pads and etched recess) of a large chip is designed and optimized to operate under similar current density with respect to smaller contemporary LEDs, so that the heat generation rate per unit area is about the same in both instances. In general, the large size chip is operated at about 250-350 milliamps and 3.5 volts.
When designing a large size device, one needs to pay extra attention to how current will spread in the device, so that a minimum of current crowding occurs. Such current crowding occurs because the lateral resistance increases with the chip dimension and because current tends not to travel to far laterally before traveling vertically. When this happens, sufficient electrical contact distribution needs to be provided so as to ensure that substantially all of the lateral-current is evenly distributed. In actual practice, an inter-digital finger pattern is used, such as the one shown in FIG. 2.
Even though large size devices can deliver more output power per device, there is a light extraction problem associated with large dimension devices. When light is generated in an LED, some light escapes easily from the chip and some light doesn't, depending on the angle at which the light impinges upon the interface between the LED and the outside media. When the optical index of refraction of the media is smaller than the index of the LED material, light inside the LED will be reflected back to the LED when the angle of incidence is greater than a critical angle. The reflected light then bounces inside the LED until it finds a way out or is absorbed. The light intensity attenuates due to absorption in the bulk material.
The more light travels inside the LED and the larger the index of refraction of the LED material is, the less light will escape. Thus, it is desirable to minimize the number of bounces and the total travel distance before light can escape for any light transmissive layer of an LED.
Referring now to FIG. 3A, as the device size increases, light has a tendency to bounce more and thus travel a longer distance before exiting the device, resulting in increased light loss. FIG. 3B shows that light tends to bounce fewer times in a smaller device and thus travels a shorter distance. This is an especially serious issue for AlInGaN on sapphire LEDs, since both AlInGaN and sapphire have relatively higher indices of refraction when compared to other LED materials such as GaAs and AlInGaP (for infrared and red to amber colors). Because of the high index of refraction of AlInGaN and sapphire, a large portion of light actually travels in these two regions and escapes from the sides. Therefore, the lateral dimension of the device is an important consideration for AlInGaN LEDs.
One remedy to this problem involves mounting the chip upside down and providing a mirror coating on the epitaxial side, so as to redirect the light output thereof toward the substrate side. Since the index of refraction of sapphire (n=1.7) is lower than that of AlInGaN (n=2.5), sapphire provides a good index matching between the AlInGaN LED and the media (n=1.5 for most epoxy). The mirror coating on the epitaxial side reflects light toward the substrate. This design provides a better pathway for light to escape therefrom. In the actual practice, light efficiency is twice that of the non-flipped large size LED. However, the cost to make this device is high since not only a sub-mount is required between the chip and the final package, but also a precise alignment is needed to ensure proper electrical contact between the chip contact pads and the sub-mount. So far, there is no evidence that such fabrication can be a high yield process.
In the device shown in FIG. 2, there are two issues with respect to light extraction. Besides the light which escapes from the device without bounces, the remaining light basically travels in the epitaxial layer region and the substrate region before finding an exit. Most of the light exits from the sides of the device. For light traveling in the substrate region, it suffers the same problems as discussed above with reference to FIG. 3A. That is, the light attenuates due to bulk and interface absorptions. This is true for both directions, i.e., parallel and perpendicular to the fingers.
For light traveling in the epitaxial layers, the light loss due to the large dimension is only along the parallel direction to the fingers. Since the distance light travels perpendicular to the fingers is much shorter before impinging upon the two sides, light escapes relatively easily with much less loss as compared to the other, longer, direction.
However, light which exits from one of the sides could hit the metal finger and then be absorbed thereby. Unfortunately, the most commonly used metals, such as Au or an Au based alloy, readily absorb light in the blue spectrum.
In actual practice, as depicted in FIG. 1 and FIG. 2, the light extraction efficiency of the 1000×1000 micron device is only ⅓ of that of the 300×300 micron device, due to the issues discussed in detail above. Because of this, even though the device area of the 1000×1000 micron chip is more than 10 times of the 300×300 micron chip, the output power is only about 3-4 times.
As such, although the prior art has recognized, to a limited extent, the problems of insufficient illumination and poor efficiency, the proposed solutions have, to date, been ineffective in providing a satisfactory remedy. Therefore, it is desirable to provide an LED having increased optical output power and enhanced efficiency. More particularly, it is desirable to provide an LED having a larger active surface, so as to provide increased brightness and efficiency with respect to contemporary LEDs.